Large pore-size molecular sieves are much in demand for reactions or separations involving large molecules since the novel family of mesoporous molecular sieves designated as M41S was discovered by Mobil Corporation scientists. These mesoporous materials, with well-defined pore sizes of 10–100 Å, overcome the pore diameter constraint (<12 Å) of microporous zeolites and offer many new possibilities in the catalytic conversion of large molecules.
The potential of mesoporous molecular sieves as catalysts has been demonstrated in a number of areas, due to the fact that the extremely high specific surface areas are conducive to high catalytic activity. The large pore size allows for the fixation of large active complexes, reduces diffusional restriction of reactants, and enables reactions involving bulky molecules to take place in the pores. The first catalytic studies with mesoporous molecular sieves focused on metal-substituted MCM-41™ materials in which the active species were incorporated into the silicate matrix. The reactions studied were mainly oxidation reactions and acid catalyzed reactions. The next stage of the development of MCM-41™ based catalysts involved the deposition of heteroatoms onto the surface of the mesoporous framework.
Another area where mesoporous materials are beneficial is separation and adsorption. The uniform pore structure within the mesopore range and the resulting high pore volume yields materials for separations that vary from the removal of organic and inorganic contaminants in waste streams to chromatographic media. Functionalized mesoporous materials have been used for the removal of heavy metals from waste streams; for example, MCM-41™ functionalized with a mercaptopropylsilane demonstrated a high affinity to extract mercury and other heavy metals from both aqueous and nonaqueous waste streams. Modification of the pore walls by coating the mesoporous structure can also alter the adsorption and catalysis behaviors of the materials. Coated materials such as a polyethylenimine coated material demonstrated excellent selectivity and high static adsorption capacity in the separation of acidic nucleotides. This static capacity demonstrated by the MCM-41™ material was higher than a comparable coated material prepared with amorphous silica.
Although extensive research efforts have been undertaken to explore the applications of mesoporous silica materials, the synthetic procedures are not as commercially viable as they might be, due to the use of high cost silicon alkoxides such as tetraethyl orthosilicate as silica sources and of hydrochloric acid which is not compatible to the stainless steel used in industrial reactors. These features are undesirable for use in commercial production.
Kim et al. have proposed in Chem. Commun., 2000, 1159–1160 to synthesize mesoporous silica materials using sodium metasilicate, hydrochloric acid and nonionic bloc copolymers as structure-directing agents. The mesoporous materials obtained were found to be of poor quality being irregular in shape and to exhibit broad pore size distributions and low surface areas.